This invention relates to ink jet printheads and more particularly to droplet-on-demand ink jet printheads having magnetically actuated means for ejecting ink droplets.
The droplet-on-demand type of ink jet printheads are generally categorized by the means used to eject the ink droplets; viz., thermal ink jet or bubble jet, piezoelectric ink jet, and acoustic ink jet. In thermal ink jet, a water based ink is used and a heating element adjacent a nozzle momentarily vaporizes the ink in contact with the heating element in response to electric pulses applied to the heating element. Once a vapor bubble is nucleated, the vapor bubble expansion and contraction initiates a drop ejection process which continues independently of any additional electrical control signals, and thus there is no mechanism for control of the drop volume as might be desirable for variable drop-size greyscale control, except for varying the printhead or ink temperature which is difficult to control. For an example of thermal ink jet printheads, refer to U.S. Pat. No. 4,638,337. The piezoelectric ink jet printheads have piezoelectric devices which expand or contract when an electric signal is applied to produce the pressure required to eject a droplet or refill the chamber. Unlike the thermal ink jet drop ejector, the expansion and contraction of the chamber volume of a piezoelectric printhead is under continuous electrical control, which allows for controlling the drop volume enabling variable drop-size greyscale printing. For an example of a piezoelectric printhead, refer to U.S. Pat. No. 4,584,590. An acoustic ink jet printhead requires the use of an RF power supply to generate the acoustic energy necessary to eject a droplet. Such an RF power supply is costly and can lead to undesirable RF emissions. The acoustic energy must be tightly focused on the ink surface in order to eject an ink droplet, which leads to very tight tolerances in the design of the printhead, and makes the printhead difficult to manufacture. For an example of an acoustic ink jet printhead refer to U.S. Pat. No. 4,751,530.
Current thermal ink jet printheads require about 5-10 .mu.J of energy supplied over a 2.7 .mu.sec time period, and thus 3.5 Watts of power, in order to eject a 20 pL droplet at 10 m/sec. Such a droplet would have 1 nJ of kinetic energy and 0.2 nJ of surface energy, and thus 99.98% of the drop ejection energy goes into waste heat. The thermal inefficiency of thermal ink jet printheads leads to a number of performance limitations; e.g., thermal management becomes a major issue and this problem gets larger as the arrays of nozzles increase. There are also problems with heat management with respect to image quality. As the thermal ink jet printhead heats up, the properties on the ink change (e.g., ink viscosity), leading to changes in the ejected droplet size, thus affecting image quality. Another limitation on thermal ink jet printheads is the restriction to water based inks, because a water vapor bubble is used as the propellant for the ink droplets. Water based inks limit ink latitude which leads to print or image quality limitations, including image permanence, water fastness, smear, and color gamut.
Both piezoelectric ink jet and acoustic ink jet printheads avoid these limitations by using non-thermal means of ejecting droplets. While this leads to increased ink latitude and eliminates heat management problems, there are a number of other problems for each of these techniques. For the piezoelectric ink jet devices, the droplet ejector must be very large, since the piezoelectric actuators provide very little displacement, thus limiting the number of nozzles in an array and thereby affecting print quality and/or productivity. Piezoelectric droplet ejectors are currently fabricated one-by-one, using non-integrated circuit batch fabrication techniques, so that their cost per nozzle is very expensive relative to droplet ejectors fabricated by integrated circuit batch fabrication techniques, such as that used by thermal ink jet devices. Acoustic ink jet printing requires the use of a RF power supply to generate the acoustic energy necessary to eject an ink droplet, and such RF power supplies are expensive. The RF power distribution on the droplet ejector heads is difficult to control. In addition, acoustic ink jet devices use non-standard fabrication processes and materials, with mechanical tolerances on the order of micrometers in all three dimensions which must be uniform over large areas, and thus do not benefit from the economies of silicon or integrated circuit batch fabrication techniques.
An electro-mechanically actuated ink jet printhead is disclosed in the article entitled "An Ink Jet Head Using a Diaphragm Microactuator," by Susumu Hirata et al, Proceedings of the Ninth Annual International Workshop on Micro Electro Mechanical Systems, San Diego, Calif., February 1996, pgs. 418-423. This device uses heat to expand and deform a diaphragm to eject ink droplets. The required energy was 80 .mu.J and is less energy efficient than thermal ink jet devices which use about 10 .mu.J.
U.S. Pat. No. 5,402,163 discloses an ink jet printhead which uses an electric current conductive ink and a current conductive bar to create an electro-dynamic force to eject ink droplets. However, this device requires a current conductive ink and thus has limitations on ink latitude, among other disadvantages.
U.S. Pat. No. 4,983,883 discloses an ink jet printhead which uses a magnetic force generating member to act upon a magnetic ink to eject droplets. Since the ink must be magnetic, this requirement imposes serious limitations on ink latitude, among other disadvantages of such a printhead.
U.S. Pat. No. 4,845,517 discloses an ink jet printhead in which a conductive mercury thread is positioned in each ink channel and a magnetic field is applied orthogonally to the channel. A flow of current through the thread causes an electromagnetic deformation of the thread and thereby eject a droplet. An apparent limitation on this concept is the exposure of the ink to the mercury thread which would lead to ink latitude problems.
U.S. Pat. No. 4,620,201; U.S. Pat. No. 4,633,267; and U.S. Pat. No. 4,544,933 disclose a magnetic driver for an ink jet printing device in which many current loops, each with a discharge nozzle, are lying in a common ink chamber. The current loops are moveable under the influence of a magnetic field and act to displace droplets. However, since the current loops act on a common ink chamber, there can be interactions between the different current loops, thus leading to cross talk between droplet ejectors. In addition, since the chamber walls in this design are very distant from the nozzles, and there are low compliance gaps between the nozzles, the mechanical efficiency of the current loops for ejecting liquid droplets is limited.
U.S. Pat. No. 4,455,127 discloses a compact size plunger pump in which pistons are driven to reciprocate by a plunger associated with an electromagnetic solenoid. Since this concept uses an electromagnetic solenoid, it does not lend itself to integrated circuit batch fabrication technology, thus this concept it not economically practical for use in an ink jet printhead environment.
U.S. Pat. No. 4,415,910 discloses an ink jet droplet ejector in which pressurized ink is released on demand by action of an electromagnet operating to unseat a magnetic ball seated on a printhead nozzle. This concept uses a magnetically actuated valve which is not suitable for integrated circuit batch fabrication technology and, thus, this concept is not considered economically practical for use in an ink jet printhead environment.
U.S. Pat. No. 4,057,807 and U.S. Pat. No. 4,032,929 disclose an ink jet printhead comprised of a plurality of ink chambers, each with a nozzle, each chamber has a diaphragm as an outer wall, and an electromagnet which may be selectively energized confronts each diaphragm. When exposed to a magnet field, the diaphragm deforms to decrease the chamber volume and eject a droplet from the nozzles. This concept is not amenable to the silicon integrated circuit batch fabrication technology, so that it is not very cost effective to manufacture, nor is it amenable to the microelectromechanical technology which is so important in a practical, cost effective ink jet printing device.